Remove unnecessary casts of NULL

Remove unnecessary casts of NULL

I noticed some unnecessary casts of NULL.  This removes all the
unnecessary ones, leaving only ones where we must ensure that NULL has
pointer type for passing through varargs.

I removed a couple of useless casts of 0 that I noticed while writing
this.

Tested by rebuilding.

gdb/ChangeLog
2019-06-16  Tom Tromey  <tom@tromey.com>

	* valops.c (value_cast, value_slice): Remove unnecessary cast.
	* breakpoint.c (stopin_command, stopat_command)
	(until_break_command, decode_location_default): Remove unnecessary
	cast.
	* utils.c (subset_compare): Remove unnecessary cast.
	* ada-lang.c (ada_update_initial_language): Remove unnecessary
	cast.
	* linespec.c (decode_line_with_last_displayed): Remove unnecessary
	cast.
	* infcmd.c (path_command): Remove unnecessary cast.
	* coffread.c (decode_type): Remove unnecessary cast.
	* xcoffread.c (read_xcoff_symtab): Remove unnecessary cast.
	* mipsread.c (mipscoff_symfile_read): Remove unnecessary cast.
	* tui/tui-stack.c (tui_show_locator_content)
	(tui_show_frame_info): Remove unnecessary cast.
	* tui/tui-win.c (tui_scroll_forward_command)
	(tui_scroll_backward_command, tui_set_focus, tui_set_win_height)
	(parse_scrolling_args): Remove unnecessary cast.
	* tui/tui-data.c (init_win_info, tui_del_window)
	(tui_free_window, tui_del_data_windows, tui_free_data_content)
	(free_content_elements): Remove unnecessary cast.
	* tui/tui-windata.c (tui_first_data_item_displayed): Remove
	unnecessary cast.
	* tui/tui-source.c (tui_set_source_content)
	(tui_vertical_source_scroll): Remove unnecessary cast.
	* tui/tui-layout.c (tui_default_win_height): Remove unnecessary
	cast.
	* tui/tui-io.c (tui_initialize_io): Remove unnecessary cast.
	* tui/tui-regs.c (tui_display_registers_from)
	(tui_display_register): Remove unnecessary cast.
	* tui/tui-wingeneral.c (tui_refresh_win, tui_delete_win)
	(tui_unhighlight_win, tui_highlight_win, tui_make_window)
	(make_visible): Remove unnecessary cast.
	* tui/tui-winsource.c (tui_erase_source_content)
	(tui_update_breakpoint_info, tui_set_exec_info_content): Remove
	unnecessary cast.
	* ax-gdb.c (agent_command_1): Remove unnecessary cast.
	* cli/cli-setshow.c (cmd_show_list): Remove unnecessary cast.
	* stabsread.c (read_type, read_array_type, read_range_type):
	Remove unnecessary cast.
	* mdebugread.c (mdebug_build_psymtabs): Remove unnecessary cast.
	(parse_symbol, parse_type, upgrade_type, parse_external)
	(parse_partial_symbols, psymtab_to_symtab_1, cross_ref): Remove
	unnecessary cast.
	* gdb_bfd.c (gdb_bfd_map_section): Remove unnecessary cast.